It has been recognized that the T1/E1 rate (1.544/2.048 Mbit/s) is a cost effective way of user access to an ATM network as well as connection between ATM network switches. However, as ATM technology for wide area networks is deployed more and more, demands for transmission links of a rate higher than T1/E1 are increasing. Links of higher rates, such as T3/E3 (44.736/34.368 Mbit/s), have been designed to meet these needs. However, the cost of T3/E3 links is still prohibitive in many cases and the ratio of cost versus realistic utilization of the entire rate is not always attractive and fully justified for new ATM end users and service providers. IMA (short for inverse multiplexing for ATM) satisfies the need by using multiple T1/E1 links which are grouped collectively to provide the service at a higher rate.
FIG. 1 and FIG. 2 show two sample configurations in which IMA is used. In this specification, inverse multiplexing for ATM and ATM devices performing inverse multiplexing are collectively called IMA. FIG. 1 depicts a user access to a network through user network interfaces (UNIs) and FIG. 2 illustrates a link connection between ATM switches through broadband inter-carrier interfaces (BICIs) or private network to network interfaces (PNNIs).
Referring to the figures, the basic function of IMA device is to work in pairs to take an ATM cell stream coming from the ATM layer, send it over the multiple links by spreading cells over the available links and ensure that the initial cell stream can be retrieved at the far end. Thus the IMA preferably makes the ATM traffic transparent to the ATM layer over multiple links in use. As far as the ATM layer is concerned, it should only see a pipe (can be considered as a virtual link) whose rate is now the sum of the multiple link rates. It is assumed that each link is run in clear-mode without the presence of intermediate ATM nodes processing ATM cells. This means that there should be no cell discard by any intermediate transmission equipment.
U.S. Pat. No. 5,608,733, Mar. 4, 1997, Vallee et al uses ATM sequence number cells indicating a specific round robin order of a plurality of transmission links over which ATM data cells are transmitted. The ATM sequence number cells also indicate whether or not a destination is ready to receive ATM data cells in that specific round robin order.
The present invention extends further a variety of functionalities which are useful in inverse multiplexing.